Radio spectra-polarimetric properties of TXS 0025+197 (J0028+2000)

ATel #12193; F. K. Schinzel (NRAO), R. Ojha (NASA/GSFC/UMBC), L. Sjouwerman (NRAO)
on 8 Nov 2018; 17:30 UT
Credential Certification: Frank Schinzel (fschinze@nrao.edu)

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A rapid optical and gamma-ray flare of TXS 0025+197 (Stanek et al. ATel #12082; Buson et al. ATel #12084) accompanied by a decay in X-rays (Komossa et al. ATel #12094) were reported at the beginning of October, 2018. TXS 0025+197 (J0028+2000) is a known, compact radio source detected at mas scales. On 2013-01-09 it was observed as compact/unresolved object at 2.3 GHz and showed a slight extension to the South-West at 8.6 GHz, with an observed peak brightness of 0.253 Jy/beam and 0.245 Jy/beam respectively (see http://astrogeo.org/vlbi/solutions/rfc_2018c/rfc_2018c_cat.html ).

Following the reported fast optical flare and related gamma-ray activity, we obtained spectra-polarimetric information between 1.0 and 47 GHz with NRAO's Very Large Array. The first observation was conducted on October 25th, 2018 (UT 05:06 - 06:42) in D-array configuration. Unfortunately, this observation was compromised by bad high frequency observing conditions and flux densities were not reliable above 20 GHz. The observation was repeated on November 2nd, 2018 (UT 04:54 - 06:30) under much improved atmospheric conditions. The obtained values for TXS 0025+197 are consistent between both observations for 1 to 20 GHz. The observations provide a spatial resolution of about 15 - 0.5 arcsec for 1.0 - 47 GHz respectively. Throughout the covered frequency range and spatial resolutions, TXS 0025+197 was detected as an unresolved point source at all bands. A subset of the observed values are listed below, with better sampled data available upon request. Errors are statistical only, systematic calibration errors affecting absolute flux density are increased above 15 GHz.

The radio spectrum is flat with no sign of a rise at the high frequency end (S(f) = 238 mJy (f/25.0)^(-0.100+-0.563*(f/25.0)), where f is the observing frequency in GHz). Current observed flux density values are consistent with historic measurements at 2-4 and 13-17 GHz in the first half of 2017. The new observations of TXS 0025+197 show linear polarization fractions ranging from a few percent polarized between 1 and 4 GHz after which a rapid drop is observed depolarizing around 6 GHz. Above 6 GHz, the polarization fractions recover and reach up to 4% around 40 GHz. At least two different rotation measure (RM) features are found across the broad bandwidth. One has a RM = -4236 +/-658 rad/m^2 (10 - 47 GHz), while a second has RM = 34 +/-10 rad/m^2 (1-5 GHz). This points to multiple emission regions as suggested through earlier milliarcsecond-scale imaging of the object. At this point there is no indication of an increase in activity below 47 GHz. We plan to obtain at least one more observation with the VLA to evaluate changes in TXS 0025+197, especially at the highest frequencies. We will report if there are significant changes in brightness or polarization observed.

# Frequency Flux_Density Lin.Pol.Fraction Pol.Angle
# (GHz) (mJy) (%) (deg.)
1.424 345.9 +/-1.6 1.2 +/-0.1 20.5 +/-2.0
1.808 350.6 +/-0.7 1.1 +/-0.1 -80.9 +/-1.6
2.691 343.4 +/-1.6 2.8 +/-0.1 -71.9 +/-0.5
3.307 338.2 +/-3.7 2.1 +/-0.1 -68.8 +/-0.7
4.935 331.9 +/-2.4 0.63 +/-0.10 -76.6 +/-3.7
5.807 333.5 +/-2.1 0.25 +/-0.10 -78.8 +/-8.2
8.563 323.2 +/-1.8 0.92 +/-0.10 0.5 +/-2.4
9.435 319.8 +/-4.7 1.1 +/-0.1 2.7 +/-1.9
13.563 304.5 +/-5.0 2.2 +/-0.1 -4.1 +/-3.2
14.564 296.4 +/-6.9 2.2 +/-0.1 -5.9 +/-1.0
19.264 285.2 +/-5.0 2.1 +/-0.1 22.9 +/-1.4
20.624 285.9 +/-6.7 2.4 +/-0.1 24.6 +/-1.3
31.564 189.8 +/-4.1 2.7 +/-0.2 56.6 +/-2.3
33.564 162.5 +/-3.8 2.6 +/-0.3 69.8 +/-2.9
35.436 169.7 +/-3.5 3.0 +/-0.2 75.7 +/-3.5
37.308 153.6 +/-4.8 2.4 +/-0.3 85.4 +/-4.6
41.065 153.8 +/-2.1 4.0 +/-0.4 -69.1 +/-2.9
43.065 134.1 +/-3.8 4.0 +/-0.6 -52.9 +/-3.7
44.937 135.5 +/-3.1 4.8 +/-0.6 -39.0 +/-5.0
46.937 119.1 +/-1.8 3.7 +/-0.9 -21.5 +/-8.4